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IEEE 802.11n

IEEE 802.11n is a proposed amendment to the IEEE 802.11-2007 wireless networking standard to significantly improve network throughput over previous standards, such as 802.11b and 802.11g, with many experts claiming that this wireless technology will finally allow consumers to move beyond traditional wired ethernet LANs.

Description

IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO) and 40 MHz operation to the physical (PHY) layer. MIMO uses multiple transmitter and receiver antennas to improve the system performance. The 40 MHz operation uses wider bands, compared to 20 MHz bands in previous 802.11 operation, to support higher data rates. Wider bandwidth channels are cost effective and easily accomplished with moderate increases in digital signal processing.

If properly implemented, 40-MHz channels can provide greater than two times the usable channel bandwidth of two 802.11 legacy channels. Coupling MIMO architecture with wider bandwidth channels offers the opportunity of creating very powerful yet cost-effective approaches for increasing the physical transfer rate. MIMO can provide many benefits, all derived from the ability to process spatially different signals simultaneously. Two important benefits explored here are antenna diversity and spatial multiplexing. Using multiple antennas, MIMO technology offers the ability to coherently resolve information from multiple signal paths using spatially separated receive antennas.

Multipath signals are the reflected signals arriving at the receiver some time after the original or line of sight (LOS) signal has been received. Multipath is typically perceived as interference degrading a receiver's ability to recover the intelligent information. MIMO enables the opportunity to spatially resolve multipath signals, providing diversity gain that contributes to a receiver's ability to recover the intelligent information.

Another valuable opportunity MIMO technology may provide is Spatial Division Multiplexing (SDM). SDM spatially multiplexes multiple independent data streams, transferred simultaneously within one spectral channel of bandwidth. MIMO SDM can significantly increase data throughput as the number of resolved spatial data streams is increased. Each spatial stream requires its own TX/RX antenna pair at each end of the transmission. MIMO technology requires a separate radio frequency chain and analog-to-digital converter for each MIMO antenna. This increasing complexity ultimately translates to higher implementation costs as higher-performance systems are required.

A second technology being incorporated into 802.11n is channel bonding, which can use two separate non-overlapping channels at the same time to transmit data. This technique also increases the amount of data that can be transmitted. A third technology in 802.11n is called payload optimization or packet aggregation, which, in simple terms, means more data can be stuffed into each transmitted packet.

Data encoding

The transmitter and receiver use precoding and postcoding techniques, respectively, to achieve the capacity of a MIMO link. Precoding includes spatial beamforming and spatial coding, where spatial beamforming improves the received signal quality at the decoding stage. Spatial coding can increase data throughput via spatial multiplexing and increase range by exploiting the spatial diversity, through techniques such as Alamouti coding.

Number of Antennas

The number of simultaneous data streams is bounded from above by the minimum number of antennas in use on both sides of the link. However, the individual radios often further limit the number of spatial streams that may carry unique data. The moniker helps identify what a given radio is capable of. The first number (a) is the maximum number of transmit antennas or RF chains that can be used by the radio. The second number (b) is the maximum number of receive antennas or RF chains that can be used by the radio. The third number (c) is the maximum number of data spatial streams the radio can use. For example, a radio that can transmit on two receive antennas, receive on three, but only send or receive two data streams would be 2x3:2.

The 802.11n draft allows up to 4x4:4. However, the Wi-Fi Alliance 802.11n Draft 2.0 certification only allows for two data spatial streams. Common configurations of Wi-Fi Alliance Draft 2.0 certified devices are 2x2:2, 2x3:2, and 3x3:2. All three configurations have the same maximum throughputs and features, and differ only in the amount of diversity the antenna systems provide.

Frame Aggregation

The main medium access controller (MAC) feature that provides a performance improvement is aggregation. (Refer to a PPT presentation by IEEE, which shows the performance of the various 802.11n MAC features at the completed proposal stage. Since then, some of the details may have changed, but features and performance are essentially unchanged.) Two types of aggregation are defined:

1. Aggregation of MAC service data units (MSDUs) at the top of the MAC (referred to as A-MSDU aggregation)
2. Aggregation of MAC protocol data units (MPDUs or frames) at the bottom of the MAC (referred to as A-MPDU aggregation)

Aggregation in the MAC is necessary to make the best use of the properties of the 802.11n PHY - i.e., while increasing the data rate, its overhead has also increased. A-MPDU aggregation requires the use of Block Acknowledgement or BlockAck, which was introduced in 802.11e and has been optimized in 802.11n. Reverse Direction is an optional feature of the 802.11n MAC that supports a bidirectional data flow given a single channel access.

Backwards compatibility

When 802.11g was released to share the band with existing 802.11b devices, it had to provide ways of ensuring coexistence between the legacy and the new devices. Now 802.11n extends coexistence management to protect its transmissions from legacy devices, which include 802.11g, 802.11b and 802.11a.

802.11n has three differences in the type of protection it enables.

1. Wi-Fi Alliance 11n Draft 2.0 devices often operate in "mixed mode". In mixed mode, each 802.11n transmission is always embedded in an 802.11a or 802.11g transmission. For 20MHz transmissions, this embedding takes care of the protection with 802.11a and 802.11g. However, 802.11b devices still need CTS protection.
2. 40MHz transmissions in the presence of 802.11a, 802.11b, or 802.11g clients require protection with a CTS on both 20MHz sides of the 40MHz channel, to prevent legacy devices from being interfered with.
3. An access point may also advertise for devices to use CTS or RTS/CTS protection, even with mixed-mode transmissions.

Even with protection, the increased throughput of 802.11n can cause large discrepancies between the throughput an 802.11n device can achieve when alone compared to what it can get when legacy devices are present. This is an extension of the 802.11b/802.11g coexistence problem.

Status

Work on the 802.11n standard dates back to 2004. The draft is expected to be finalized in November 2008 with publication in July 2009, but major manufacturers are now releasing 'pre-N', 'draft n' or 'MIMO-based' products based on early specs. These vendors anticipate the final version will not be significantly different from the draft, and in a bid to get the early mover advantage, are pushing ahead with the technology. Depending on the manufacturer, a firmware update may eventually be able to make current "Draft-N" hardware compatible with the final version.

Wi-Fi Alliance

As of mid-2007, the Wi-Fi Alliance has started certifying products based on IEEE 802.11n Draft 2.0. This certification program established a set of features and a level of interoperability across vendors supporting those features, thus providing one definition of 'draft n'. The certification covers both 20 MHz and 40 MHz wide channels, and up to two spatial streams, for maximum throughputs of 130Mbps for 20 MHz and 300Mbps for 40 MHz. A number of vendors, in both the consumer and enterprise spaces, have built products that have achieved this certification. The Wi-Fi Alliance certification program subsumed the previous industry consortium efforts to define 802.11n, such as the now dormant Enhanced Wireless Consortium (EWC). The Wi-Fi Alliance is investigating further work on certification of additional features of 802.11n not covered by the Draft 2.0 certification, including higher numbers of spatial streams (3 or 4), as well as extended range support through beamforming and Space-Time Block Coding

Timeline

January 2004

IEEE announced that it had formed a new 802.11 Task Group (TGn) to develop a new amendment to the 802.11 standard for wireless local-area networks. The real data throughput will reach a theoretical 270 Mbit/s for the required dual stream MIMO device. (which may require an even higher raw data rate at the physical layer), and should be up to 20 times faster than 802.11b, up to 3 times faster than 802.11a, and up to 4 times faster than 802.11g.

July 2005

Previous competitors TGn Sync, WWiSE, and a third group, MITMOT, said that they would merge their respective proposals as a draft. The standardization process is expected to be completed by the second quarter of 2009.

19 January 2006

The IEEE 802.11n Task Group approved the Joint Proposal's specification, based on EWC's draft specification.

March 2006

The IEEE 802.11 Working Group sent the 802.11n Draft to its first letter ballot, allowing the 500+ 802.11 voters to review the document and suggest bugfixes, changes and improvements.

2 May 2006

The IEEE 802.11 Working Group voted not to forward Draft 1.0 of the proposed 802.11n standard. Only 46.6% voted to approve the ballot. To proceed to the next step in the IEEE standards process, a majority vote of 75% is required. This letter ballot also generated approximately 12,000 comments—much more than anticipated.

November 2006

TGn voted to accept draft version 1.06, incorporating all accepted technical and editorial comment resolutions prior to this meeting. An additional 800 comment resolutions were approved during the November session which will be incorporated into the next revision of the draft. As of this meeting, three of the 18 comment topic ad hoc groups chartered in May have had completed their work and 88% of the technical comments had been resolved with approximately 370 remaining.

19 January 2007

The IEEE 802.11 Working Group unanimously (100 yes, 0 no, 5 abstaining) approved a request by the 802.11n Task Group to issue a new Draft 2.0 of the proposed standard. Draft 2.0 was based on the Task Group's working draft version 1.10. Draft 2.0 was at this point in time the cumulative result of thousands of changes to the 11n document as based on all previous comments.

7 February 2007

The results of Letter Ballot 95, a 15-day Procedural vote passed with 97.99% approval and 2.01% disapproval. On the same day, 802.11 Working Group announced the opening of Letter Ballot 97. It invited detailed technical comments to closed on 9 March 2007.

9 March 2007

Letter Ballot 97, the 30-day Technical vote to approve Draft 2.0, closed. They were announced by IEEE 802 leadership during the Orlando Plenary on 12 March 2007. The ballot passed with an 83.4% approval, above the 75% minimum approval threshold. There were still approximately 3,076 unique comments, which will be individually examined for incorporation into the next revision of Draft 2.

25 June 2007

The Wi-Fi Alliance announces its official certification program for devices based on Draft 2.0.

07 September 2007

Task Group agrees on all outstanding issues for Draft 2.07. Draft 3.0 is authorized, which possibly may go to a sponsor ballot in November 2007.

November 2007

Draft 3.0 was approved (240 voted affirmative, 43 negative, and 27 abstained). The editor was authorized to produce draft 3.01.

January 2008

Draft 3.02 was approved. This version incorporates previously approved technical and editorial comments. There remain 127 unresolved technical comments. It is expected that all remaining comments will be resolved and that TGn and WG11 will subsequently release Draft 4.0 for working group recirculation ballot following the March meeting.

CSIRO controversy

In late November 2007, work on the 802.11n standard slowed due to patent issues. The CSIRO holds the patent to a component of the 802.11n standard. This component is also part of 802.11a and 802.11g. The IEEE requested from the CSIRO a Letter of Assurance (LoA) that no lawsuits would be filed for anyone implementing the standard. The CSIRO responded that, as they were currently being sued by some of the companies who would be sheltered by the LoA, they could not provide one at this time without adversely impacting their defense in the lawsuits.

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